The use of petroleum based monomers in the manufacture of consumer products is expected to decline in the coming years because of the continuous rise in the price of oil and the high rate of depletion of known oil reserves. This, in connection with strict government regulations all around the world on environmental protection against pollution, has inspired the investigation of renewable resources as a possible alternative to petroleum based monomers. With the diminishing of the limited petroleum resources, use of renewable resources as chemicals for industrial applications is of great interest. Vegetable and animal oils, typically used as a food source for human beings, represent a major class of such resources and are being increasingly used for industrial applications. These oils have many desirable characteristics. For example, they are non-toxic, biodegradable, and environmentally friendly, and in some applications may prove to be more cost effective compared to petroleum based oils.
The chemistry and physics of natural materials encompass some of the most challenging and complex issues facing modern science. Raw materials from renewable resources are rich in chemical reactivity, stereochemical diversity and physical structure presenting great potential for industrially useful products. Natural plant oils also known as triglycerides contain active sites that allow chemical reactions. Those active sites are the double bonds, hydroxyl groups along the hydrocarbon chains, and ester groups. These active sites can be used to introduce polymerizable groups, increase the molecular weight of the starting materials via polymerization of the double bonds, or introduce glycols or organic acids to form polymers.
Currently, triglycerides such as soy oil, corn oil, and olive oil are commonly used in food applications. Industrial applications of these oils include coatings, inks, plasticizers, lubricants and agrochemicals. Conversion of triglycerides with a variety of polyfunctional glycols is well know via the transesterification of the fatty esters and further reaction with diacids and anhydrides to form alkyd resins. The architecture of these resins is such that the backbone constitutes a polyester composed of polyols and polycarboxylic acids. To this backbone, fatty acids from one or more drying or semi-drying oils are covalently linked. The “curing” or crosslinking takes place by air mediated peroxidative mechanisms in the presence of metal salts or drying agents through the fatty acid unsaturation. This process requires intimate contact with air; however, these products can be used as thin films with limited structural strength. Additionally, difficulties are encountered when trying to homopolymerize the double bonds of the fatty acids. This is due to the facile chain transfer of the allylic nature of the double bonds in the fatty acid chains.
Conventional alkyds are characterized by rather high molecular weight and broad molecular weight distributions, which stem from their production procedures. Modifications in the process conditions can cause large discrepancies in the properties of the manufactured materials. In addition to the variation of properties, gelation can take place, which can be partly suppressed by reducing the ratio between the polycarboxylic monomers and the polyols. This becomes more critical when maleic anhydride is introduced in the reaction mixture of compositions containing triglycerides. Maleic anhydride can undergo Diels-Alder and ene reactions that increase the branching of the system and therefore cause gelation.
In the composite industry, the most common resins used for closed and open molding processes are unsaturated polyesters. Several papers and patents are found in the literature describing reactive systems containing renewable plant base raw materials for Sheet Molding Compounds (SMC) and Open mold applications. Uses of triglycerides include mold release agents as fatty acid amides intermediates and fatty acid salts. Examples can be found in U.S. Pat. Nos. 4,144,305, 5,576,409, 5,744,816, and 5,883,166. Epoxidized fatty oils have been used as additives in SMC applications and also modified with polybasic acids to produced vinyl ester components for molding products. Examples are described in U.S. Pat. Nos. 6,900,261, 4,367,192, 5,504,151, and 2,949,441.
In the modification of fatty oils with maleic anhydride in these types of products, the maleic anhydride reacts with the triglyceride via Diels-Alder and ene reactions to provide oligomeric intermediates. These intermediates are modified with acrylates to provide materials capable of reacting via free radical polymerization using organic peroxides. Examples of these intermediates are presented in U.S. Pat. No. 6,121,398, Japanese Patent No. 81-64464, and German Patent No. 89-3938149.
Preparation of unsaturated polyesters modified with triglycerides has been reported in the literature. Bakare et al., in J. Appl. Polym. Sci. 100, 3748 (2006), describe polyesters prepared from fatty oils that find applications in surface coatings and composites. The amount of maleic anhydride used in the preparation of the intermediates is low, and the main reaction is with the unsaturation of the fatty acid. Eren, in J. Appl. Poly. Sci. 91,4037 (2004), modified an unsaturated fatty acid by first hydromethylating the double bonds of the oil and then reacting the hydroxyl groups with maleic anhydride. The resulting materials are diluted in styrene monomer and crosslinked with a peroxide at room temperature. Penczek et al., in Fatipec Congr. 2, 617(2004), describe unsaturated polyesters prepared from fatty oils in combination with DCPD, maleic anhydride and glycols. The resulting materials had good physical properties although low glass transition temperatures.
U.S. Patent Publication Application No. 2003/0092841 describes low molecular weight dimer acids used as LPA surface quality enhancers. These materials are used in combination with unsaturated polyesters in SMC applications. U.S. Pat. No. 6,222,005 describes the preparation of unsaturated polyester resins by first end-capping a carboxylic acid or its corresponding anhydride with a saturated monohydric alcohol to form a half ester and then reacting the half ester with a polyol and an unsaturated fatty oil.
In the many applications in which molding compositions have been used, one of the major shortcomings is the overall physical strength of the composition as compared to steel, aluminum, and other materials which compete in the markets. In order to compete, articles made of SMC and BMC usually require increased thicknesses to improve impact strength and physical properties such as Tensile and Flexural strength. Resins can be tailored to improve toughness and thermal properties of the molded articles. The balance between these two properties is primarily dictated by the crosslink density within the resin. The greater the crosslink density, the higher the heat deflection temperature and glass transition temperature but the lower the toughness and impact resistance. Toughness and impact resistance improvements can also be achieved by the addition of flexible moieties, such as ether groups into the polymer backbone, although at the sacrifice of thermal properties. Thus, it is desirable to improve the toughness and impact resistance of the resin system without significantly reducing its thermal properties.
A significant disadvantage of prior art products is that they have rough and undulating surfaces exhibiting a characteristic pattern of the reinforcing fibers. The rough surfaces are attributable, at least in part, to the shrinkage in volume, which occurs as the resin polymerizes. While this may not be the only factor contributing to the poor surface smoothness on the moldings, it is thought to be a predominant factor. To overcome the surface roughness and to reduce volume shrinkage, unsaturated polyester technology often employs low profile agents. In order for the low profile agents to perform effectively, highly reactive unsaturated polyester resins are generally required. This high reactivity results in cured resins with very high crosslink densities that are brittle in nature. Toughness in polymer compositions comes about with materials of low crosslink density with high elongations at failure. These tougher materials usually exhibit low glass transition temperatures, low heat distortion temperatures, a low resin modulus and low strength.